Chapter 13 Review: Darwin, Evolution and Natural Selection

Ch 13 ReviewEvolutionDarwin &Natural Selection

Darwin

In 1859, in London, an English naturalist named Charles Darwin proposed a reasonable theory that explained that species evolved over time.

Darwin published his evidence from his decades of observations he made travelling the globe as a naturalist. Darwin’s book was called “The Origin of Species”

Before Darwin, most people, including scientists, held the view that each species was a divine creation of God that exists unchanged from it’s first creation.

A handful of scientists proposed that organisms changed over time and tried to propose mechanisms of how this would happen.

In 1809, French scientist Jean Baptiste Lamarck proposed a hypothesis for how organisms change over generations. Lamarck believed that , over the lifetime of an individual, physical features decrease in size because of disuse or increase because of use.

Lamarck believed these changes were than passed down to offspring.

While Lamarck’s hypothesis in now known to be incorrect, he did first identify one crucial truth: that change in a species is linked to the physical conditions of life.

During Darwin’s voyage around the globe, he found evidence that challenged the traditional beliefs that species wee unchanging. As Darwin travelled and collected evidence, he found fossils that suggested that creatures were slowly changing over time.

Fossil’s from South America were similar to modern species but there were subtle differences showing that change had occurred over long periods of time.

Darwin’s key breakthroughs came when he visited the Galapagos islands off the coast of South America. Darwin noticed that many animals and plants resembled those from the South American coast 1000 miles away.

Megatherium: The size of an African bull elephant, with enormous claws on its feet, twenty feet tall when standing:

Modern day sloth

Darwin suggested that the simplest explanation for this was that the Galapagos species were the ancestors of species from South America that had migrated to the island and changed after they arrived. Darwin called this change evolution.

It was many years after Darwin’s return from his voyage before he put together all his evidence. Darwin knew that evolution occurred but was puzzled about how it did so.

Darwin’s thinking about evolution came together when he read an essay by English economist Thomas Malthus. Malthus wrote about human populations and pointed out that human populations are able to grow faster than their food supply. His work compared geometric population growth to arithmetic population growth. Malthus pointed out that populations grow geometrically, faster than the food supply, which grows arithmetically.

He also suggested that human populations do not grow unchecked; that war, disease, and famine slow population growth.

A population consists of all individuals of a species that live in a specific geographical area and can interbreed.

A population may be all the species of tree frogs that live in the Amazon rainforest while another population may be all the species of penguins that live in Antarctica.

Darwin realized that Malthus’s hypothesis about human populations apply to all species.1. Every organism has the potential to

reproduce many offspring over it’s lifetime.2. In most cases, however, only a limited

number of those offspring survive to reproduce.

3. Those that have some type of advantage to survive are most likely to reproduce.

Darwin came to a key conclusion: Individuals that have physical or behavioral traits that

better suit their environment are more likely to survive will reproduce more successfully than those without

the traits.

Darwin called this differing rate of survival-reproduction

natural selection. ____________________________________________

Natural selection is the process by which individuals that have favorable variations and are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do.

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In time, the number of individuals that carry favorable characteristics that are inherited (passed along to them) will increase in population because of this adaptation. This process is called evolution.

An adaptation is an inherited trait that has become common in a population because the trait provides a selective advantage.

Publication of Darwin’s Work: In 1859, Darwin published his book “On the

Origin of Species by Means of Natural Selection”. Many people were deeply disturbed by Darwin’s theory, including the suggestion, that humans are related to apes.

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Darwin’s Theory of Evolution by Natural Selection is supported by 4 major points:1. Inherited variation exists within the genes of

every population or species2. In a particular environment, some individuals

of a population or species are better suited to survive and thus have more offspring.

3. Over time, the traits that aid certain individuals of a population in survival and reproduction tend to spread in that population, being passed on to their offspring.

4. There is overwhelming evidence from fossils and many other sources that living species today evolved from organisms that are extinct.

Green beetles taste good thusbrown beetles survive

Publication of Darwin’s Work: Darwin’s key idea was based on the concept that

in any population, individuals that are best suited to survive and do well in their environment will live longer and produce the most offspring.

Certain forms of a trait become more common in a population because more individuals in a population carry the alleles for that trait. Natural selection causes the frequency of certain alleles in a population to increase or decrease over time.

Environments differ from location to location. Thus, populations of the same species living in different locations tend to evolve in different ways. They all evolve toward what gives them the best chance of survival in their unique location.

Reproductive isolation is the condition in which two populations of the same species do not breed with one another because of a geographical separation.

The Tempo of Evolution:

For decades, biologists have understood evolution as a gradual process that continuously occurs.

Gradualism is the name we give to the model of evolution in which gradual change over time leads to a species formation.

American biologists Stephen Jay Gould and Niles Eldredge have suggested that successful species may remain unchanged for long periods of time . They have hypothesized that major environmental changes in the past have caused evolution to occur in spurts.

This model of evolution, in which periods of rapid change are separated by periods of little or no change, is called punctuated equilibrium.

Fossil Records Evolution Fossils provide a record of past life forms and

their stages of evolution. The age of the fossil shows where that organism was at that particular stage of it’s development to what it now is today.

Fossils of animals show a pattern of development, from early ancestors to modern descendents. Fossils are the most direct evidence that evolution takes place. Fossils therefore provide an actual record of Earth’s past life-forms.

After observing differences, Darwin predicted that intermediate forms between the groups of organisms would eventually be found. Since Darwin’s time, some of the intermediate steps have been found; some have not.

For example, intermediate steps have been found between fishes and amphibians, between reptiles and birds, and between reptiles and mammals.

Today, scientists agree that Darwin’s theory is the best available explanation for the biological diversity on Earth.

Based on a large body of supporting evidence, most scientists agree on the following three major points

1. Earth is about 4.5 billion years old2. Organisms have inhabited Earth for most of it’s history3. All organisms living today share common ancestry with earlier

simpler life-forms

The fossil record, and thus the record of life on Earth, is not complete. Many species have lived in environments where fossils do not form. Most fossils form when organisms are buried in fine sediments deposited by wind, water, or volcanic eruptions.

The environments that are most likely to cause fossil formation are wet lowlands, slow-moving streams, lakes, shallow seas, and areas near volcanic eruptions. The chances that organisms living in upland forests, mountains, grasslands, or deserts will die in the right spot to be fossilized is very low.

Because of these reasons, fossil records will never be entirely complete. Fossils still present us with the best evidence that evolution has taken place however.

When a fossil is discovered, paleontologists (scientists who study fossils) analyze the sediments around it. By radiometric dating techniques (measuring half lives of surrounding rocks), paleontologists can arrange the fossils in order from oldest to youngest. When this is done, orderly patterns of change through evolution can be seen clearly.

At right we see the proposed evolution of the modern whale. Scientists have pieced this evolution together by looking at series of fossils and trying to place them in an order that shows the development.

Anatomy and Development: Comparisons of the anatomy of different types of

organisms often reveal basic similarities in body structures even though the structures functions may differ between organisms.

Sometimes bones are present in an organism but are reduced in size and either have no use or a less important use than is found in other related organisms. These structures are considered to be evidence of an organism’s evolutionary past, and are called vestigial structures.

Homologous Structures: As different groups of vertebrates evolved, their bodies evolved differently. But similarities in bone structure can still be seen, suggesting all vertebrates share a common ancestor. In the diagrams at right, you can see that the forelimbs of vertebrates are composed of the same basic groups of bones. We call these structures homologous. Homologous structures are structures that share a common ancestry.

Vestigial hip bones in modern whales showthat whales descendedfrom creatures thatonce walked on land

Natural selection at Work:

The core of Darwin’s Theory of Evolution is that natural selection is the mechanism that drives evolution.

The key lesson that modern scientists have learned about natural selection is that the environment dictates the direction and amount of change.

If the environment changes in the future, than the characteristics that help an individual organism survive and reproduce successfully may change.

The process of natural selection is driven by 4 important factors:

1: All populations have genetic variation: In any population, there is an array of individuals that differ slightly from one another in genetic makeup.

2: The environment presents challenges to successful reproduction. Naturally, an organism that does not survive to reproduce or whose offspring die before the offspring can reproduce does not pass on it’s genes to future generations.

3: Individuals tend to produce more offspring than the environment can support. Thus individuals of a population often compete with each other for food, shelter, and breeding partners. Those best suited to attract a mate, have a better chance to breed and pass on genes.

4: Individuals that are better able to cope with the challenges presented by their environment tend to leave more offspring.

Darwin collected 31 specimens of finches from three islands when he visited the Galapagos Islands. In all, he collected 9 distinct species, all very similar to one another except for their bills.

Two ground finches with large bills feed on seeds that crush in their beaks, while two with narrower bills eat insects. One finch is a fruit eater, one picks insects out of cactus, while yet another creeps up on seabirds and uses it’s sharp beak to drink their blood.

Darwin suggested the nine species of finches all developed from an original ancestral species. Changes occurred as different populations adapted to different food sources. In years when food was not as plentiful, those finches that had adaptations that aided them to get food in their immediate location survived and bred where their counterparts perished.

The formation of species occurs in stages. Natural selection favors changes that increase reproductive success. Therefore, species molded by natural selection has an improved “fit” to it’s environment. The accumulation of differences between groups is called divergence. Divergence leads to the formation of new species.

Biologists call the process by which new species form speciation.

Separate populations of a single species often live in several different kinds of environments. In each environment, natural selection acts upon the population. Natural selection results in offspring that are better suited to that environment.

If the environments differ enough, separate populations of the same species can become very dissimilar

Over time, populations of the same species that differ genetically because of adaptations to different living conditions become what scientists call a subspecies. The members of a newly formed subspecies have taken the first step toward speciation.

Eventually the subspecies become so different that they can no longer interbreed successfully. Biologists than consider them a separate species.

What keeps new species separate? Why are even closely related species usually unable to interbreed? Once subspecies become different enough, a barrier to reproduction usually prevents different groups from breeding with each other.

Black rattlesnake

There are several types of barriers that may isolate two or more closely related groups. Groups may be geographically isolated or may reproduce at different times. Physical differences may also prevent mating, or they may not be attracted to one another for mating. The hybrid offspring of a mating may not be fertile or may not be suited to the environment of either parent.

Scientists have seen the stages of speciation in many different organisms. The ways that natural selection leads to the formation of new species has been thoroughly documented. As changes continue to build over time, living species may become very different from their ancestors and from other species that evolved from the same recent common ancestor, leading to the formation of a new species.

Black rattlesnake